[media] coda: v4l2-compliance fix: implement try_decoder_cmd

Implement try_decoder_cmd to let userspace determine available commands
and flags.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>